The present invention relates to a receiving layer transfer sheet, and more specifically to a receiving layer transfer sheet used to form a color image of high quality by thermal transfer recording methods.
Among various thermal transfer methods so far proposed and known in the art, there is a method for forming various full-color images on a thermal transfer image receiving sheet such as paper or plastic film including a dye receiving layer thereon, using a thermal transfer sheet made up of a substrate film such as paper or plastic film on which a sublimable dye is carried as a recording agent. This method uses a printer's thermal head as heating means, and enables a multiplicity of three- or four-color dots to be transferred onto the thermal transfer image receiving sheet by very short heating, thereby reproducing a full-color image of the original with said multi-color dots.
The thermal transfer image receiving sheet that enables image formation to be achieved by the method mentioned above is limited to a dyeable plastic sheet, or a paper or other sheet with a dye receiving layer preformed thereon; that is, there is a grave problem that any image cannot directly be formed on generally available plain paper or other sheets.
Of course, images may be formed even on general plain paper, provided that it has a receiving layer on the surface. However, this does not only cost much but also has difficulty in application to generally available materials onto which images are to be transferred, for instance, postcards, letter paper, and pads for note-taking, reporting and other purposes.
One known approach to solving these problems is to use a receiving layer transfer sheet designed to enable a dye receiving layer to be easily provided only on the necessary portion of an associated or cooperative member made of generally available plain paper or other material. In this regard, see JP-A 62-264994.
Another approach known for additional convenience is to make use of a receiving layer transfer sheet made up of a continuous substrate film having yellow, cyan and magenta dye layers possibly with a black dye layer formed successively on the surface in this order and in the longitudinally direction, and including a dye receiving layer on the same film surface. The receiving layer is first transferred onto a cooperative member, and the respective dye layers are subsequently transferred onto the receiving layer to form a full-color image. Still another receiving layer transfer sheet having on the surface a protective layer for protecting the formed dye image is proposed as well.
However, all conventional receiving layer transfer sheets have problems each awaiting solution, as set forth below.
(a) The transferable resin layer of one typical conventional receiving layer transfer sheet is essentially required to include a dye receiving layer and an adhesive layer laminated on its substrate sheet surface, on which additional layers such as release and intermediate layers are co-laminated as occasion demands. At least one layer contains foams or a foaming agent for the purpose of improving image quality. However, the incorporation of foams or a foaming agent gives rise to increases in the heat insulation and total thickness of the transfer layer, which otherwise makes the transmission of transfer heat unsmooth, often resulting in a transfer failure. When this receiving layer transfer sheet is used to transfer the transferable receiving layer onto a postcard, plain paper or other substrate sheet, the transfer function depends primarily on the adhesion properties of the adhesive layer. For instance, by use of an adhesive material having a relatively low softening temperature it is possible to achieve a pattern form of transfer as by a thermal head with relatively low energy. However, the use of an adhesive material having a low softening temperature offers a blocking problem when the receiving layer transfer sheet is stored, especially in a rolled-up state. When used within the printer over an extended period of time, this blocking problem becomes serious due to a temperature increase within the printer.
These problems may be easily solved by use of an adhesive material having a relatively high softening temperature. When a thermal head is used for transfer, however, excessive heat energy is needed, making the durability of the thermal head worse. In addition, this adhesive material fails to produce its own adhesion at the initial stage of transfer at which the interior temperature of the printer remains low, often resulting in transfer defects such as "chipping" or "dropping-out" of a transferred layer. As the interior temperature of the printer increases with time, on the other hand, another problem arises; that is, the surface of the dye receiving layer to be transferred comes to having fine asperities corresponding to the printing pressure of the thermal head.
Therefore, the first object of the present invention is to provide a receiving layer transfer sheet which is free from any blocking problem during storage or in use and enables good-enough transfer to be achieved by application of relatively low energy, and the transferability of which is not affected by a temperature change in a printer, thus making homogeneous transfer and the formation of high-quality images possible.
(b) Usually, the transferable resin layer of another typical conventional receiving layer transfer sheet is essentially required to include a dye receiving layer and an adhesive layer laminated on its substrate sheet surface, on which additional layers such as releasing, barrier and foamed layers are co-laminated as occasion demands. The transferable resin layer is transferred by various techniques onto a suitable cooperative member such as paper. When the resin layer is transferred onto the cooperative member in a desired pattern by means of heating with a thermal head, however, it is required that the resin layer be precisely transferred in a desired form.
It is here to be noted that the intermediate layer such as a foamed layer and the adhesive layer forming part of the transferable resin layer are made of resins of low strength, but the dye receiving layer is made of a thermoplastic resin of relatively high strength and heat resistance for the purpose of obtaining image stability upon image formation. This makes it impossible to achieve transfer of a precise pattern by a thermal head, and so offers a grave problem; that is, the transferred resin layer have zigzag edges or, in the alternative, it has some transfer defects represented by "tailing".
Therefore, the second object of the present invention is to provide a receiving layer transfer sheet which, upon transfer, can be cut off by a thermal head in a precise and easy manner and enables high-quality images to be formed.
(c) Usually, the transferable resin layer of the typical conventional receiving layer transfer sheet is essentially required to include a dye receiving layer and an adhesive layer laminated on its substrate sheet surface, on which additional layers such as release, barrier and foamed layers are co-laminated as occasion demands. In most cases, these layers are formed by the coating and drying of coating solutions having a suitable resin dissolved in an organic solvent. Ideally, all the solvents must be evaporated off upon drying. Usually, however, perfect removal of the solvents is not industrially advantageous if the solvent composition, ability-to-be-coated, productivity, and other factors of the coating solutions are taken into account; that is, it is unavoidable that some amounts of the solvents remain in the transferable resin layer. It is of course preferable that such solvent residues are reduced as much as possible. In other words, a problem associated with the properties and productivity of coating solutions is to what degree the amount of solvent residues is acceptable.
Too much solvent residues offer many problems. For instance, the receiving layer cannot be cut off upon transfer, the receiving layer cannot be transferred in a precise form, the receiving layer is transferred with "tailing" and with poor releasability, the receiving layer fuses and sticks fast to the dye layer of a thermal transfer sheet upon image formation after transfer, and the formed image degrades with time. Especially in the case of a composite thermal transfer sheet including a dye layer, solvent residues migrate into the dye layer, having an adverse influence on the dye of the dye layer.
As a matter of course, the problems mentioned above may be solved by imposing strict limitations on the conditions for drying coating solutions after coating. However, such a solution is industrially unpractical, because a very time-consuming drying procedure is needed, and because much difficulty is involved in foaming control when microcapsules in the foamed layer are kept unfoamed.
Therefore, the third object of the present invention is to provide a receiving layer transfer sheet which, while the amount of solvent residues in a transferable resin layer and productivity are well balanced, and enables the transferable resin layer to be well transferred without offering the problems mentioned above even in the presence of solvent residues, thereby making the formation of high-quality images possible.